Stem Cells
Stem cells are characterized in that they are capable of self renewal (cell division without differentiation) and also of producing progeny that are more differentiated. The quintessential stem cell historically is the embryonic stem (ES) cell. The ES cell has unlimited self-renewal. ES cells are derived from the inner cell mass of the blastocyst or primordial germ cells from a post-implantation embryo (embryonal germ cells or EG cells). ES and EG cells have been derived, among others, from mouse, non-human primates and humans. When introduced into blastocysts, ES cells can contribute to all tissues. A drawback to ES cell therapy is that when transplanted in post-natal animals, ES and EG cells generate teratomas.
ES (and EG) cells can be identified by positive staining with antibodies to SSEA1 (mouse) and SSEA4 (human). At the molecular level, ES and EG cells express a number of transcription factors specific for these undifferentiated cells. These include oct3/4 and rex-1. Also found are the LIF-R (in mouse) and the transcription factors sox-2 and rox-1. Rox-1 and sox-2 are also expressed in non-ES cells. A hallmark of ES cells is telomerase enzyme activity, which provides these cells with an unlimited self-renewal potential in vitro. See, for example, U.S. Pat. Nos. 5,453,357; 5,656,479; 5,670,372; 5,843,780; 5,874,301; 5,914,268; 6,110,739 6,190,910; 6,200,806; 6,432,711; 6,436,701, 6,500,668; 6,703,279; 6,875,607; 7,029,913; 7,112,437; 7,145,057; 7,153,684; and 7,294,508, each of which is incorporated by reference for teaching ES cells and methods of making them. ES cells have been grown in aggregate form. They are able to form embryoid bodies when grown without attachment to a substrate.
Oct3/4 (oct3 in humans) is a transcription factor expressed in the pregastrulation embryo, early cleavage stage embryo, cells of the inner cell mass of the blastocyst, and in embryonic carcinoma (EC) cells (Nichols et al., Cell 95:379-91 (1998)), and is down-regulated when cells are induced to differentiate. Expression of oct3/4 plays an important role in determining early steps in embryogenesis and differentiation. Oct3/4, in combination with rox-1, causes transcriptional activation of the Zn-finger protein rex-1, also required for maintaining undifferentiated ES cells (Rosfjord and Rizzino, Biochem Biophys Res Commun 203:1795-802 (1997); Ben-Shushan et al., Mol Cell Biol 18:1866-78 (1998)). In addition, sox-2, expressed in ESC/EC, but also in other more differentiated cells, is needed together with oct3/4 to retain the undifferentiated state (Uwanogho et al., Mech Dev 49:23-36 (1995)). Maintenance of murine ES cells and primordial germ cells requires the presence of LIF. The oct3/4 gene is transcribed into at least two splice variants in humans, oct3A and oct3B. The oct3B splice variant is found in many differentiated cells whereas the oct3A splice variant (also previously designated oct3/4) is reported to be specific for the undifferentiated ES cell. See Shimozaki et al. Development 130:2505-12 (2003).